1. Field of the Invention
The present invention generally relates to a balun, and more particularly, to a balun including a plurality of metallic layers.
2. Description of Related Art
A balun (balanced-unbalanced transformer) is a device for transforming a balanced signal to an unbalanced signal or transforming an unbalanced signal to a balanced signal. The balanced signal is made up of two signals that are nearly 180 degrees out of phase with each other and have nearly equal amplitudes. The balun usually has two balanced terminals for receiving and outputting balanced signals and one unbalanced terminal for receiving and outputting unbalanced signals.
As shown in FIGS. 1A and 1B, U.S. Pat. No. 5,497,137 discloses a conventional balun 100 having a thin layer laminate 100a. The thin layer laminate 100a includes an earth electrode 110a, a connecting electrode 110b, a first strip line 110c, a second strip line 110d, an earth electrode 110e, a first dielectric substrate 120a, a second dielectric substrate 120b, a third dielectric substrate 120c, a fourth dielectric substrate 120d, and a fifth dielectric substrate 120e. 
When fabricating such the laminate 100a, firstly the earth electrode 110a is formed on the first dielectric substrate 120a; the connecting electrode 110b is formed on the second dielectric substrate 120b; the first strip line 110c is formed on the third dielectric substrate 120c; the second strip line 110d is formed on the fourth dielectric substrate 120d; and the earth electrode 110e is formed on the fifth dielectric substrate 120e. Subsequently, the substrates are superimposed to configure the laminate 110a. Because the laminate 110a is configured by a superimposing process, each of the electrodes and the strip lines demands a dielectric substrate corresponding thereto, and thus the laminate 100a is relatively thick and has many layers.
The first strip line 110c includes a first spiral line 112 and a second spiral line 114 which are connected to each other. The first spiral line 112 has a width 112a smaller than a width 114a of the second spiral line 114. An inner end of the first spiral line 112 is electrically connected to the connecting electrode 110b via a conductive via 130a configured through the second dielectric substrate 120b. An inner end of the second spiral line 114 is an open-circuited end.
The second strip line 110d includes a third spiral line 116 and a fourth spiral line 118 which are independently provided. The third spiral line 116 corresponds to the first spiral line 112, and the fourth spiral line 118 corresponds to the second spiral line 114. An inner end of the third spiral line 116 is electrically connected to the earth electrode 110e via a conductive via 130b configured though the fourth dielectric substrate 120d. An inner end of the fourth spiral line 118 is electrically connected to the earth electrode 110e via another conductive via 130c configured through the fourth dielectric substrate 120d. 
The balun 100 further includes eight external electrodes 140a, 140b, 140c, 140d, 140e, 140f, 140g, and 140h, configured at side surfaces of the laminate 110a. The external electrodes 140a, 140d, 140e, and 140h are electrically connected to the earth electrodes 110a, 110e. The external electrode 140b is electrically connected to an outer end of the third spiral line 116. The external electrode 140c is electrically connected to an outer end of the fourth spiral line 118, and the external electrode 140f is electrically connected to an end of the connecting electrode 110b which is far from the conductive via 130a. 
Referring to FIGS. 1B and 1C, a distance between each two opposite sides of an innermost circle of the first spiral line 112 is defined as a first distance 112b, and a distance between each two adjacent parallel line segments of the first spiral line 112 is defined as a second distance 112c. Each of the second distance 112c is approximately equivalent to one of the first distances 112b. A distance between each two opposite sides of an innermost circle of the second spiral line 114 is defined as a third distance 114b, and a distance between each two adjacent parallel line segments of the second spiral line 114 is defined as a fourth distance 114c. Each of the third distance 114c is approximately equivalent to one of the fourth distances 114b. As such, while the external electrode 140f serves as an input electrode and the external electrodes 140b, 140c serve as output electrodes, the balun 100 has higher return loss and insertion loss. Further, the conventional balun 100 has a poor quality factor Q. Briefly, the conventional balun 100 has a poor electrical performance.